Process for purifying hydrogen sulphide containing hydrogen cyanide



Dec. 24, 1957 J. D. F. MARSH ET AL PRocEss FOR PURIFYINGHYDROGEN-SULPHIDE mi2; EQ2 @2:08

H NN 5,22 w z3 o o@ United States PROCESS FOR PURIFYING HYDROGENSULPHIDE CONTAINING HYDROGEN CYANIDE John David Francis Marsh andWilliam Barry Savile Newling, London, England, assignors to North ThamesGas Board, London, England, a British body corporate This invention isfor improvements in or relating to the purification of hydrogen sulphideand has particular reference to the removal of hydrogen cyanide fromgases rich in hydrogen sulphide, e. g. from gases containing more thanof hydrogen sulphide by volume calculated on the dry basis.

Gases to which the present invention is applicable include particularlythe concentrates of hydrogen sulphide obtained from the regenerativestage of cyclic liquidreagent processes for the removal of hydrogensulphide from coal gas or coke-oven gas. The ratio of hydrogen cyanideto hydrogen sulphide by volume in such gases may be as little as 1:12 oras much as 1:3.

The hydrogen sulphide concentrates are most commonly of value if theycan be converted either to pure sulphur, as in the Claus process, or tohigh-grade sulphuric acid in a suitable form of sulphuric acid Contactplant. In either case, the presence of hydrogen cyanide is undesirableas the products of its oxidation may contaminate the resulting sulphuror sulphuric acid or may be objectionable in the waste gases.

The methods hitherto applied in commercial practice for the removal ofhydrogen cyanide from gases rich in hydrogen sulphide involved therecovery of the hydrogen cyanide as such or in the form of alkalicyanides, ferrocyanides, or thiocyanates. In many cases the provision ofthe expensive equipment and the elaborate safety measures needed torecover and handle the hydrogen cyanide or other cyanogen compounds isnot desirable and it is preferred to destroy the hydrogen cyanide byconversion to products permitting easier disposal.

We have now found that the destruction of hydrogen cyanide may beeffected in the presence of excess hydrogen sulphide by hydrolysing thehydrogen cyanide with water vapour in the presence of a contactcatalyst, the nitrogen content of the hydrogen cyanide being convertedto ammonia which is easily disposable.

According to the present invention there is provided a process forremoving hydrogen cyanide from gases containing more than 10% by volumeon the dry basis ot hydrogen sulphide which process comprises reactingthe gaseous mixture with water vapour at a temperature above 150 C. andnot exceeding 500 C. in the presence of a contact catalyst andthereafter removing the resulting ammonia by washing.

The process is preferably carried out above 200 C. in order to ensure anelfective rate of reaction. The preferred temperature range is between200 C. and 400 C. At temperatures in excess of 500 C. the equilibriumfor the reaction does not permit of removal of the hydrogen cyanide withsufficient completeness.

In the present invention an alumina catalyst is employed and this may beor may initially take the form of an oxide, hydroxide or partiallydehydrated hydroxide. These are either themselves active catalysts orare converted to active catalysts in the reaction chamber by contactwith the gas being treated and under the normal aan ice

conditions of operation of the process e. g. by partial dehydration orby interaction with the components of the gas.

One preferred form of alumina is the gamma form of the oxyhydroxide ormetahydroxide of aluminium, v-AlODH. This may be made by partialdehydration of the hydroxide Al(CH)3. A commercial material, availableas activated alumina and used in the drying of gases, contains a largeproportion of Iy-AlOOH; this activated alumina is particularlyconvenient and effective as a catalyst for the present invention.

Another preferred form of alumina is the gamma oxide y-Al203. This maybe prepared by the dehydration of ^/AlO.OH by known methods e. g. byheating to a temperature within the range 500 to 700 C. For example, acommercial activated alumina, if calcined by heating at 600 C. for 30minutes, may be used as a catalyst in the present invention at atemperature about 25 C. lower than would be needed to give the sameperformance with the parent, uncalcined, commercial activated alumina.The particle size of the catalyst may conveniently be from 3/16" to 1%for a rate of ow of gas of the order of 4-000 cu. ft. per hour.

The primary products of the reaction are ammonia and carbon monoxide.Some of the carbon monoxide may react further with any excess of watervapour to form carbon dioxide and hydrogen. A small quantity of carbonoxysulphide may also be formed by the reaction of the oxides of carbonwith the hydrogen sulphide. These additional products are notobjectionable.

The water vapour content of the gaseous mixture to be treated may beadjusted to the required excess over the hydrogen cyanide by theaddition of steam or by bringing the gaseous mixture into contact withwarm water of the appropriate vapour pressure. When the gases to betreated in the present invention are derived from the regenerative stageof a cyclic liquid reagent process for the removal of hydrogen sulphidefrom coal gas or colte oven gas adjustment of the water vapour contentmay be effected by partial cooling and condensation of the excess watervapour.

The gas to be purified is preferably heated to the temperature requiredat the inlet to the cataiyst chamber, e. g. to about 250 C. This may beeifected by external heating with gas burners or by internal heatingwith electric heaters. The reaction is accompanied by a temperature risewhich, in an adiabatic system, is of the order of 10 C. to 12 C. foreach 1% by volume of hydrogen cyanide destroyed.

The throughput of gas in the catalyst bed is preferably within the rangeto 1000 standard cu. ft. per hour per cu. ft. of space occupied by thecatalyst.

For any given set of catalyst temperatures, the degree of hydrogencyanide conversion Varies markedly with the hydrogen cyanide content ofthe ingoing gas.

To attain the same percentage conversion of hydrogen cyanide to ammoniaand carbon monoxide without altering the catalyst temperature, thethroughput of gas must be adjusted approximately in inverse. proportionto the hydrogen cyanide concentration in to ingoing gas. For example, ifconditions are such that with an untreated gas containing 8% HCN byvolume the conversion is 99% at 600 standard cu. ft. per hour per cu.ft. of catalyst space, then for a gas containing 24% HCN the throughputfor 99% conversion will be about 200 catalyst space volumes per hour ifother conditions are substantially similar.

Removal of the resulting ammonia may conveniently be carried out byfirst cooling the gas and the-n washing with cold water. When the gastreated is a product of a coalgas purification process, the etduentwashing liquor may asiat-tuo conveniently bea'dded to the gasworksammoniacal liquor. Alternatively, the ammonia mayl be lremoved by firstcooling and then washing the treated gas with sulphuric acid.

Following is a descriptionby way of example and with reference to theaccompanying drawing of methods ernbodying the present invention.

Referring to the drawing:

The single gure is a flow sheet indicating the several Steps in thetreatment of a gaseous mixture containing hydrogen cyanide.

Example l Referring to the drawing, la gaseous mixture, the product of aliquid-reagent process for the recovery of hydrogen sulphideconcentrates from coal gas, containing by volume 50.4% hydrogensulphide, 5.2% hydrogen cyanide, 34.3% water vapour, 8.9% carbon dioxideand 1.2% nitrogen is fed into a cooler 11 at a flow rate of about 18.05pound molecules per hour, a temperature of about 124 C. Iand a gaugepressure of about 2 lbs. sq. in.

In the cooler 11 the gaseous mixture is brought below the dewpoint ofabout 72 C. by contact with water-cooled tubes so that the greater partof the water vapour is condensed and separated and the smaller partremaining in the gases which leave the cooler at a temperature of about47 C. is about 50% greater in volume than the hydrogen cyanide present.

After this adjustment of the water vapour content the gaseous mixturepasses to the upper part of a preheater 12. In the preheater the gaseousmixture passes downward through a number of tubes heated on the outsideby the products of combustion of fuel gas from a burner 13. The quantityof gas burnt in the burner is controlled by a thermostat 14 to ensurethat the hydrogen sulphide-containing gas mixture is passed at atemperature of about 300 C. from the preheater direct into a reactionvessel for the catalytic hydrolysis of its hydrogen cyanide content. Theinner part of the reaction vessel is a catalyst basket 15 in the form ofa cylinder about 38" high and of 24 internal diameter. The catalystbasket is externally flanged and open at the top and contains a bed ofan alumina catalyst i6 with a depth or about 36 supported on a gridforming the base of the catalyst basket 15. The catalyst 16 takes theform of porous pieces substantially of A1202 and within the size range%G%. The catalyst has been made from a commercial activated alumina byheating in such manner that it was within the temperature range 580-600C. for one hour only.

The outer part of the catalyst vessel 1'? is a thermally insulatedcylinder about 42 high and of 28" internal di ameter closed at the baseand internally flanged and with an oftake pipe 18 near the top. lThehydrogen sulphidecontaining gas mixture is passed from the pre-heaterdownward through the catalyst bed and returns upwards through theannulus between the catalyst basket I and the outer part of the catalystvessel 17 'and then out via the pipe 18 to a cooler 19, T he maximumtemperature of the catalyst is about 370 C. and the temperature at thepipe 18 is about 330 C. The approximate composition of the gas at thepipe i8 is 68.65% H23, 7.00% NH2, 0.015% HCN, 13.00% CO2, 6.10% CO, 0.9%H2, 1.65% N2 and 2.68% water vapour. The coo-ler 19 takes the form of aserpentine rack of tubes in series through which the gas from the pipei8 is passed upward.

Cooling water is passed downward over the outer surfaces of the tubes ofthe rack in quantity suicient to ensure that the gas is cooled to atemperature of about 25 C. just above its dewpoint before being passedto an ammonia Washer 20.

In this unit the gas is countercurrently contacted and Washed with astream of water entering from a line 2l at a temperature of about 12 C.and at a flow rate of about 70 gallons per hour. On the ceramic ringpacking 22 in the washer 20 the ammonia is absorbed by the water washand leaves as a solution containing by weight approximately 2.0% NH3,3.7% .ll-12S and 0.8% CO2. This solution is passed through a line 23 forfurther processing together with the ammonia recovered from the coal gasstream from which the hydrogen sulphide-containing concentrate has beenseparated. The treated gaseous mixture is passed out of the plant by aline 24 at a rate of 11.23 pound molecules per hour and with theapproximate cornposition 73.7% H25, 0.016% HCN, 0.002% NH3, 14.1% CO2,7.2% CO, 1% H2, 2% N2 and 2% Water vapour.

Example 2 A gaseous mixture containing by volume on the dry basis 75%hydrogen sulphide, 8% hydrogen cyanide and 17% carbon dioxide, is firstbubbled through water at about 50 C. in order to adjust the water vapourcontent of the gas to substantially 50% excess by volume over the volumeof the hydrogen cyanide. The wet gas is then heated to 300 C. and passedinto a bed of granular activated alumina, substantially 'y-AlODI-I of agrade normally used for drying air, ata rate of 1000 cu. ft. per hour,dry basis at atmospheric temperature for each cubic foot of spaceoccupied by the catalyst. The treated gas is then passed through apacked tower in countercurrent flow to a washing liquid -consisting ofcold water which removes the `ammonia and reduces the water vapourcontent of the u gas to below 2% by volume.

The resulting ammonia-free gas contains less than 0.1% by volume ofhydrogen -cyanide and is suitable for oxidation with undried air in acontact plant to yield 93% sulphuric acid low in nitrogen acids, withoutproducing waste gases having an excessive concentration of nitrogenoxides.

We claim:

1. A process for purifying hydrogen sulphide containing hydrogen cyanidewhich process comprises removing at least 99% of the hydrogen cyanidepresent in the hydrogen sulphide by reacting the gas with water vapourat a temperature between 200 and 400 C. in the presence of a catalystselected from the group consisting of the gamma form of aluminummetahydroxide and the gamma form of anhydrous alumina and thereafterremoving the resulting ammonia from the gaseous mixture by washing.

2. A. process for purifying gases containing not less than 50% by volumeon the dry basis of hydrogen sulphide and also containing hydrogencyanide which process comprises removing at least 99% of the hydrogencyanide present by reacting the gaseous mixture with water vapour at atemperature between 200 and 400 C. in the presence of a catalystselected from the group consisting of the gammal form of aluminummetahydroxide and the gamma form of anhydrous alumina and thereafterremoving the resulting ammonia from the gaseous mixture by washing.

3. A process for purifying hydrogen sulphide containing hydrogen cyanidewhich process comprises removing at least 99% of the hydrogen cyanidepresent in the hydrogen sulphide by reacting the gas with water vapourat a temperature between 200 and 400 C. in the presence of a catalystselected from the group consisting of the gamma form of aluminummetahydroxide and the gamma form of anhydrous alumina and thereafterremoving the resulting ammonia from the gaseous mixture by washing, thewater vapour content of the gas being adjusted so that it is at leastequal to the hydrogen cyanide content.

4. A process for purifying gases containing not less than 50% by volumeon the dry basis of hydrogen sulphide and also containing hydrogencyanide which process comprises removing at least 99% of the hydrogencyanide present by reacting the gaseous mixture with water vapour at atemperature between 200 and 400 C. in the presence of a catalystselected from the group consisting of the gamma form of aluminummetahydroxide and the gamma form of anhydrous alumina and thereafterremoving the resulting ammonia from the gaseous mixture .by washing, thewater vapour content of the gaseous mixture being adjusted by theaddition of steam so that it is at least equal to the hydrogen cyanidecontent.

5. A process for purifying hydrogen sulphide containing hydrogen cyanidewhich process comprises removing at least 99% of the hydrogen cyanidepresent in the hydrogen sulphide by reacting the gas with water vapourat a temperature between 200 and 400 C. in the presence of a catalystselected from the group consisting of the gamma form of aluminummetahydroxide and the gamma form of anhydrous alumina and thereafterremoving the resulting ammonia from the gaseous mixture by washing, thethroughput of gas in the catalyst bed being within the range of 100 to1000 standard cu. ft. per hour per cu. ft. of space occupied by thecatalyst.

6. A process for removing at least 99.5% of the hydrogen cyanide presentin a gaseous mixture derived from the regenerative stage of a cyclicliquid reagent process for the removal of hydrogen sulphide andcontaining more than 50% by volume on the dry basis of hydrogen sulphidewhich process comprises reacting the gaseous mixture with water vapourat a temperature between 200 and 400 C. in the presence of a catalystselected from the group consisting of the gamma form of aluminummetahydroxide and the gamma form of anhydrous alumina and thereafterremoving the resulting ammonia from the gaseous mixture by washing.

6 References Cited in the le of this patent UNITED sTATEs PATENTS943,399 Dunnackie Dec. 14, 1909 1,148,368 Evans July 27, 1915 1,498,031Halvorsen June 17, 1924 1,580,038 Halvorsen Apr. 6, 1926 1,680,807Schultze Aug. 14, 1928 1,687,118 Winkler Oct. 9, 1928 2,143,821 SperrJan. 10, 1939 2,162,838 Cole .Tune 20, 1939 2,406,420 Weiser Aug. 27,1946 2,419,225 Mitchell Apr. 22, 1947 2,432,872 Ferro Dec. 16, 19472,459,464 Smith Ian. 18, 1949 2,474,440 Smith June 28, 1949 FOREIGNPATENTS 3,148 Great Britain of 1876 231,134 Great Britain Sept. 17, 1925OTHER REFERENCES Perry: Chemical Eugineers Handbook, McGraw-Hill BookCo., New York, 1941, page 2362.

1. A PROCESS FOR PURIFYING HYDROGEN SULPHIDE CONTAINING HYDROGEN CYANIDEWHICH PROCESS COMPRISES REMOVING AT LEAST 99% OF THE HYDROGEN CYANICPRESENT IN THE HYDROGEN SULPHIDE BY REACTING THE GAS WITH WATER VAPOR ATA TEMPERATURE BETWEEN 200* AND 400*C. IN THE PRESENCE OF A CATALYSTSELECTED FROM THE GROUP CONSISTING OF THE GAMMA FORM OF ALUMINUMMETAHYDROXIDE AND THE GAMMA FORM OF ANHYDROUS ALUMINA AND THEREAFTERREMOV-VING THE RESULTING AMMONIA FROM THE GASEOUS MIXTURE BY WASHING.